Exploit-powered Android Trojan uses update attack

A new variant of the DroidKungFu Android Trojan is posing as a legitimate application update in order to infect handsets, according to security researchers from Finnish antivirus vendor F-Secure.

Distributing Android malware as updates is a relatively new tactic that was first seen in July. The primary method of infecting handsets continues to be the bundling of Trojans with legitimate applications; however, the resulting apps are easy to spot because of the extensive permissions they request at installation time.

According to security researchers, the new update-based attacks can have a higher success rate than "Trojanizing" apps, because users don't tend to question the legitimacy of updates for already-installed software.

Furthermore, when used by threats like DroidKungFu, update attacks can be hard to detect without specialized antimalware tools. That's because these Trojans use Android exploits to gain root access and then deploy their malicious components unhindered.

The new DroidKungFu variant is distributed with the help of a non-malicious application currently available from third-party app stores in China. However, the threat is global because apps infected with earlier versions of the Trojan have been detected on the official Android Market in the past.

"Once installed, the application would inform the user that an update is available; when the user installs this update, the updated application would then contain extra functionalities, similar to that found in DroidKungFu malware," the F-Secure researchers warn.

The update only asks for access to SMS/MMS messages and location, but also contains a root exploit for Android 2.2 (Froyo) that unlocks all system files and functions. Even though this particular DroidKungFu variant doesn't target devices running Android 2.3 (Gingerbread), there are other Trojans that infect this version of the operating system and could adopt the same attack technique in the future.

In addition, there is reason to believe that the malware's authors are also testing other infection methodologies. Last week, security researchers from mobile antivirus vendor Lookout detected another DroidKungFu variant that doesn't use root exploits at all.

Instead, the new Trojan, which Lookout calls LeNa, uses social engineering to trick users into giving the installer super-user access on devices where users have knowingly executed a root exploit. Once deployed, the malware attaches itself to a native system process.

"This is the first time an Android Trojan has relied fully on a native ELF binary as opposed to a typical VM-based Android application," the researchers explained. The malware is distributed by rogue VPN (virtual-private-network) applications, some of which were found on the official Android Market.

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